Before receiving radiation therapy treatment, a computed axial tomography simulation (CAT scan or CT scan) must be performed. A CT scan is used to help the radiation oncologist map the target treatment area on a patient's body. Dosimetrists and physicists plan the entire treatment from the CT images. The CT scan is performed with the patient in a comfortable, yet stable position for the area being treated. With the patient in the desired treatment position, a CT scan is performed. Using the images from the scan, an oncologist determines a treatment isocenter. When the oncologist determines the isocenter, a computer generates the coordinates for a laser projecting system. The lasers project beams from the walls on both the right and left side of the patient as well as the ceiling, indicating the three points of the coordinates. The center at which the three points intersect in the body defines the isocenter.
External marks are then placed on the patient's skin and used to align the patient for treatment each day. These external marks will indicate where the isocenter or center of treatment is within the patient's body. When the patient is aligned to these marks, prior to treatment, the isocenter will fall directly under the central axis of the radiation beam. Lining a patient up to these marks ensures that they are in the same position for treatment as they were at the time of simulation. The marks are then tattooed. These tattoos will be used during the course of treatment to reproduce how the patient was positioned during the CT scan. If a patient is having head or neck treatment these external marks are usually placed on a mask using tape and a marker and no tattoo is needed.
The current technique for tattooing external marks on a patient's skin is performed immediately after the oncologist sets the isocenter. The lasers project from the walls and ceiling making an X on the right, left, and anterior or posterior side of the patient's body. A clinician uses a marker to trace where the lasers fall on the patient's skin. The X's that are drawn on the patient's skin are then tattooed for a permanent reference point. A drop of ink is then placed on the center of the X and with a 19 gauge needle the patient is stuck, creating a permanent tattoo. A piece of gauze is used to wipe off excess ink, and additional alcohol wipes are used to remove any remaining ink on the patient's skin. This procedure is repeated for each X that is drawn on the patient's body. The process can be very time consuming, and the patient must remain still until the entire process is completed.
There are disadvantages associated with the current technique described above. For example, it is very difficult to draw a straight line on a patient's body following a thin laser. When trying to follow the laser projected on the skin, the clinician's hand tends to block the laser, making it impossible to see where the line is being drawn. Second, the marker bleeds as the lines are drawn. Additionally, a line expected to be very precise becomes 3-5 mm thick. After drawing two lines with the sharpie, it is possible to be anywhere from 0.5 cm to 1 cm off the actual center of the X. One drop of ink is then placed in the center of the X. It becomes difficult determining where the tip of a 19 gauge needle should be placed on the smeared X which was drawn with a sharpie and now has a drop of ink on top of it.
There are also disadvantages associated with the use of ink droppers for tattoos. First, when only one drop of ink should be placed on the X, often times more than one drop comes out making it difficult or impossible to see the X. Second, some patients have very dry skin which soaks up the ink making it difficult once again to see the center of the X. Additionally, about 15-20 percent of patients need to be tattooed twice because the first tattoo is not visible. Treatment tolerances and deviations from these tattoos are within millimeters. On average, the margin of error planned for a treatment is only 2-5 mm. Unfortunately, a patient may already be marked up to 1.5 cm off their original coordinates defined by the oncologist.
Thus, there remains a need for improved devices and methods for marking isocenters on a patient's body.